DIFFERENCE BETWEEN PDS AND PDMS..

First of all the database structure is totally different. PDS is a lot less user friendly, much more labor intensive than PDMS

But your asking about Piping Design specifically.

Visually PDMS is way better. You can rotate through a rendered model while designing.
In PDS you mainly see lines. and top views north view etc. rotating is possible, but you'll get lost and if the model is crowded you better dont cuz the view will take ages to refresh.
In PDS you'll get lost when working in crowded areas, and need smart plant review to see what you've actually done.PDS clash check is a disaster.
in PDMS you immediately see what you do, you can rotate around your work to see if it's done right without clashing, clash checking utility is way more user friendly.

in PDMS you can customize every thing.
also your own commands in form and buttons.

in PDS you are bound to buttons provided by Intergraph

Again PDS is much more labor intensive. If you need to reroute/modify a line in PDS deleting and starting over would be the fastest way. In PDMS you can move/rotate each component to what ever direction you want, and you're not bound to that ridiculous segment line.

I could go on for ages to name points in which PDMS is preffered over PDS.